¹J.J. Bellucci, ^1,2A.A. Nemchin, ¹M.J. Whitehouse, ¹J.F. Snape, ²P.A. Bland, ²G.K. Benedix
¹Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
²Department of Applied Geology, Curtin University, Perth, WA 6845, Australia
¹Department of Geological Sciences, Stockholm University, SE-106 91 Stockholm, Sweden

The Pb isotopic compositions of maskelynite and pyroxene grains were measured in ALH84001 and three enriched Shergottites (Zagami, RBT04262, and LAR12011) by Secondary Ion Mass Spectrometry (SIMS). A maskelynite-pyroxene isochron for ALH84001 defines a crystallization age of 4089±73 Ma (2σ). The initial Pb isotopic composition of each meteorite was measured in multiple maskelynite grains. ALH84001 has the least radiogenic initial Pb isotopic composition of any Martian meteorite measured to date (i.e., 206Pb/204Pb=10.07±0.17, 2σ). Assuming an age of reservoir formation for ALH84001 and the enriched Shergottites of 4513 Ma (Borg et al., 2003, Lapen et al., 2010), a two stage Pb isotopic model has been constructed. This model links ALH84001 and the enriched Shergottites by their similar μ-value (238U/204Pb) of 4.1-4.6 from 4.51 Ga to 4.1 Ga and 0.17 Ga, respectively. The model employed here is dependent on a chondritic μ-value (~1.2) from 4567–4513 Ma, which implies core segregation had little to no effect on the μ-value(s) of the Martian mantle. The proposed Pb isotopic model here can be used to calculate ages that are in agreement with Rb-Sr, Lu-Hf and Sm-Nd ages previously determined in the meteorites and confirm the young (~170 Ma) ages of the enriched Shergottites and ancient, >4 Ga, age of ALH84001.

Reference
Bellucci JJ, Nemchin AA, Whitehouse MJ, Snape JF, Bland P, Benedix GK (2015) The Pb isotopic evolution of the Martian mantle constrained by initial Pb in Martian meteorites. Journal of Geophysical Research, Planets (in Press)
Link to Article [DOI: 10.1002/2015JE004809]
Published by arrangement with John Wiley & Sons